The invention relates to a method for synthesizing highly electrically conductive polymers, such as polypyrroles, polythiophene, etc., electrochemically from a solvent free solid polymer electrolyte based on poly (ethylene oxide) (PEO), complexed with at least one alkali metal salt. More particularly, the invention relates to a method for synthesizing an electrically conductive polymer from a solvent-free solid polymer electrolyte which is disposed in a vacuum chamber into which monomer vapor is introduced while a predetermined electric potential is maintained across the electrolyte.
Due to their advantages of simplicity and ease of fabrication, semiconductor-electrolyte junctions have been produced by a number of innovative concepts that are designed to improve their stability and efficiency. A novel approach to the problem of stability of semiconductor electrodes has been the introduction of thin film solid polymer electrolytes. PEO based solid polymer electrolytes are of interest both for photoelectrochemical energy conversion (PEC) devices and for all solid state batteries. An advantage of such solid polymer electrolytes in PEC devices is that their inherent absence of solvent results in an absence of photocorrosion of the electrodes in the devices. In fact, it appears that the cells of such devices are stable indefinitely when care is taken to eliminate water from the solid electrolyte by suitably heating it under vacuum.
One of the problems encountered in the manufacture of such PEC devices is that they require the use of electrically conductive polymer films of about 1000 angstroms or less in thickness. It is difficult to handle such very thin films to fabricate the sandwich type cell structures used in making prior art PEC device. Thus, it would be desirable to minimize or avoid such handling by forming the thin polymer films in situ on either a metal or semiconductor electrode.
One suitable method for forming such a thin film polymer on an electrode is described in an article published by Skotheim, et al. in the Journal of the Electrochemical Society, Vol. 129, p. 894 (1982), entitled "Solid Polymer Electrolyte Photovoltaic Cell". The method described in that article involves forming a photoelectrochemical cell based on PEO-KI/I.sub.2 electrolytes, n-Si/Pt/polypyrrole photoanodes and conductive tin-oxide glass counter electrodes. The PEO-KI/I.sub.2 films in the cell were made on a conductive glass electrode by evaporation from a methanol solution at room temperature. The PEO had a molecular weight of 600,000 (Aldrich) and the concentration of potassium, as given by the stoichiometric ratio of alkali metal cations to oxygen atoms in the polymer backbone was 1/8.
The advantages of the method disclosed herein, relative to such prior art methods, would be at least two-fold: (1) by synthesizing the polypyrrole, or other electrically conducting polymer, in situ in the finished assembly one eliminates all mechanical handling of the thin films. Sometimes, if the films are intended as electronic coupling materials between electrodes and PEO electrolytes the films are so thin, e.g. about 100 Angstroms thick, that they simply cannot be handled mechanically. For optimum performance, an in situ polymerization with a PEO electrolyte can, therefore, mean the difference between practical and non-practical efficiencies in the case of such photoelectrochemical cells. (2) for other than thin film devices; by growing a film in situ one also achieves a better mechanical contact between the polypyrrole and the PEO. If the films are handled mechanically and pressed together, heating is sometimes required to a sufficiently high temperature to cause bonding to occur, and such heating of the assembly can damage it or the adjacent materials.
Accordingly, a major object of the present invention is to provide a method for forming an electrically conductive polymer film from a solvent-free solid polymer electrolyte by depositing the film directly on an ultra thin electrode that is evaporated, or otherwise deposited, onto the solid electrolyte.
Another object of the invention is to provide a method for synthesizing an electrically conductive polymer film from a solid PEO salt coating that is about 1 micron thick and that has a thin film electrode with a high density of pinholes through it formed thereon, so that monomers for forming the desired polymer film can be diffused through the pinholes in the electrode and into the PEO salt.
Yet another object of the invention is to provide a method for synthesizing thin film electrically conductive polymers in situ on a solid PEO based-salt electrolyte disposed in a vacuum chamber, while a predetermined electric potential is applied between electrodes across the electrolyte thereby to effect electrochemical oxidation of monomers from a monomer vapor introduced into the vacuum chamber, and to grow a highly conductive polymer film of a selected thickness on the electrode.
Still another object of the invention is to provide a method for synthesizing thin film electrically conductive polymers, in situ, on a solid PEO-salt electrolyte, by first absorbing pyrrole monomer vapor in a PEO-salt, while maintaining it and an associated substrate cool enough to retain the pyrrole monomers in the PEO-salt for a sufficiently long period to permit a front electrode to then be formed on the PEO-salt. The front electrode thus formed need not be ultra-thin or porous. Subsequently, a thin film of polypyrrole, (or other conductive polymer of an absorbed monomer) can be electrochemically deposited on either the substrate or the front electrode.
Additional objects and advantages of the invention will become apparent to those skilled in the art from the description of it presented herein, considered in conjunction with the accompanying drawings.